1. Field of the Invention
The present invention relates to a diaphragm-electrode system for electrocoating, and more particularly to a diaphragm-electrode system for electrocoating that serves as a counterpart electrode in a process for coating by means of electrolytic deposition, wherein a substrate (object to be coated) functions as an electrode provided on one side while the foregoing diaphragm-electrode system functions as the counterpart electrode provided on the other side.
2. Description of the Prior Art
Electrocoating is broadly classified into two catagories: one is a process using anionic paints; and another is a process using cationic paints. Either of those processes gives excellent results in homogeneity as well as in adhesiveness of the films formed over the substrates. Also, the pollution resulting from these processes is low. Therefore, recently, they are finding widespread use, particularly as the most suitable process for undercoating or one-coat finishing, etc. in the field of metal coating. For example, their application is found in automatic film treatment for automobile bodies.
Of the paints used for the electrocoating as mentioned above, as the anionic paints, for example, those which are made to be water soluble by bonding the carboxyl group to a resin of two-thousand in molecular weight are used. As the cationic paints, for example, those which are made to be water soluble by bonding amino groups to resin components of the paints are used. However, even these water soluble paints are very low in degree of ionization after they are dissolved in water. Presently, as a measure to increase the degree of ionization of the paints in water through neutralization, counteragents for neutralization are added to the paints. For example, alkaline neutralizers, such as triethylamine, are mixed in the anionic paints, while acidic neutralizers, such as lactic acid, are mixed in the cationic paints.
As is described above, neutralizers for raising the degree of ionization of the paints are mixed into the respective paints according to the nature of each of the paints. However, on the other hand, an adverse effect is also brought about from the foregoing practice. That is, as the electrodeposition for the substrates goes on and the resin components of the paint in solution gets lowered in amount, the paint must be replenished by feeding it in successively from outside. This causes a continuous acculmulation of amine or lactic acid used as neutralizer in the solution. As the result, problems, such as remelting of the painted surface (film), formation of pinholes, etc., are caused with a serious disturbance in efficiency of the electrolytic deposition as a whole.
To cope with such problems, lately, for example, so called pH control, as proposed in Japanese Patent Application publication for Opposition No. 22231 (1970), is employed for the purpose of preventing the neutralizers from building up in the aqueous solutions. The pH control works as follows. Through the use of an ion exchange membrane, etc., an electrode provided as the counterpart to the substrate that functions as an electrode on the other side is separated from the aforesaid substrate as well as from the aqueous solution. At the same time, the ion exchange membrane, etc. serves to extract amine or lactic acid from the aqueous solution through permeation, thereby preventing the accumulation of the neutralizer in the aqueous solution.
Meanwhile, the aqueous solution surrounding the substrate in a bathing tank for electrodeposition is constantly agitated in order to enhance the efficiency of electrodeposition. In addition, the water for draining out the neutralizer on the side of the counterpart electrode that is separated by means of the flat sheet form the ion exchange membrane is supplied continuously from the outside although in a very small amount. Consequently, the flat plate form ion exchange membrane mentioned above constantly receives irregular, high and low alternate pressures from both sides. The alternate pressures are applied repeatedly with an impulsive manner or with slow and gentle manner. Practically, the alternating pressure is created not only by the agitation of the aqueous solution for electrodeposition, but also by the operation of moving the substrate into or out of the bathing tank and the process of in-tank carriage of the substrate that is hung on the product line.
As to the variation in water pressure in this case, for the ordinarily used ion exchange membrane ( about 1 m in height, about 50 cm in width), if the variation in water pressure created is 0.5 kg/cm.sup.2, the variation in water pressure affecting the membrane as a whole becomes 2500 kg. This means that a tension of about 8.5 kg/cm.sup.2 is applied repeatedly to the installing portion of the membrane. Under such a situation, the ion exchange membrane (diaphragm) is continually subjected to bending action imposed to a part of it or to the whole of it. Due to the repeated impression of bedning stress and tensile force, it is impossible to use thin membranes. Even thick membranes are frequently broken within a short period of time (actually, within 2-3 days). The result is the necessity of replacing the ion exchange membrane at regular, short intervals. This in turn requires the preparation of a crane, etc. for carrying out the exchange work. In addition, the electrocoating process line itself has to be stopped.
Moreover, the following problem has also be unsolved. That is, around the counter-electrode described above, impurities which have passed through the ion exchange membrane as well as impurities in the water get attracted and polarized. Besides, due to the electrolysis of the water, bubbles are created around the counterpart electrode. These polarized particles and bubbles are impossible to be removed by the use of the conventional method of continuously trickle feeding water. Accordingly, the efficiency in electrodeposition gets lower with the passage of time. Thus, these persistent drawbacks, that is the extremely poor efficiency and accompanying high cost, have troubled the electrocoating by the use of diaphragm-electrode process based on the conventional art.
In addition to the above-mentioned various problems, there has been another problem left unsolved in electrocoating. The problem pertains to the wear of electrode rod (melting of the electrode due to electrophoresis). The work for exchanging or refilling the electrode rod requires a lot of labor.